A PET image may be a molecular picture that provides functional information such as metabolism of the body and may be helpfully used for the early detection of cancer and the treatment. Since a long time (e.g., about tens of minutes) is taken to obtain data of whole-body PET images, the scan is conducted while breathing freely. At this time, motions of chest/abdomen organs due to respirations may make the quality of PET images deteriorate due to blurring.
Meanwhile, various image distortion corrections may be subjected to construct good PET images. In case of attenuation correction, in general, correction may be made using X-ray CT images obtained by the same system (e.g., PET/CT system), just before PET image scanning. In case of a CT image, it is possible to obtain data quickly, for example, within a few seconds. For this reason, scanning may be made without breathing around a specific breathing. The attenuation correction may not be exactly performed due to a respiratory difference between the CT images and the PET image, thereby making the attenuation correction distorted. The distortion may act as a factor of hindering quantitative analysis of PET images and may make the PET images blurred. Deterioration of the quality of the PET images due to the above-described causes must be solved because hindering exact diagnosis and treatment.
To solve the above-described problems, there is widely used a respiratory gating PET imaging method that uses respiratory gating equipment upon obtaining PET images and classifies PET data according to respirations to obtain various respiratory states of PET images of which the respiratory motion is improved. However, to construct PET images of respective respirations effectively, an attenuation correction may be performed using a phase-matched CT image exactly corresponding to each respiratory state. Moreover, the quality of phase-matched PET images may be considerably lowered in terms of the signal to noise ratio (SNR). To improve the deterioration of the quality of PET images, there may be performed motion compensation in which motion information between phase-matched PET images is well estimated and other phase-matched PET images are converted into one respiration state using the estimated motion information.
As a study for phase-matched attenuation correction and respiratory motion estimation and compensation, first, there is proposed a method in which a CT image acquired using a respiratory gating CT imaging (4D CT imaging) method is used as upon acquiring PET images. This method is useful because the phase-matched attenuation correction is performed using the obtained phase-matched CT images and respiratory motion information is relatively exactly obtained from the acquired 4D images. However, the 4D CT imaging method is problematic in that radiation exposure is significantly increased as compared with the CT scanning. For this reason, the 4D CT imaging method is being rejected according to the ALARA (As Low As Reasonably Achievable) rule recommended by the ICRP from 1965. Moreover, the problem that a respiration of a phase-matched PET image is not identical to that of a phase-matched CT image frequently occurs. In this case, respiratory information obtained from the CT can be erroneously applied, thereby lowering correction and compensation effects. To correct this, there is an inconvenience that a user revises the lowering of the correction and compensation effects manually for a long time.
Another method is a method in which a CT image corresponding to one respiration is obtained as at CT acquisition of a conventional PET/CT and PET images by the respiration are used. Respiratory motion may be estimated through registration between phase-matched PET images. However, since the phase-matched PET image includes a lot of noise and activity concentration distributions of phase-matched PET images are different from each other, it is difficult to have confidence in the motion information through the registration. Also, since phase-matched PET images not experiencing the attenuation correction are low in a characteristic capable of being used for the registration, it is difficult to expect high registration performance. Also, for the phase-matched attenuation correction, it is necessary to convert the previously obtained CT image of a specific respiration according to each respiratory state using motion information obtained through the registration between phase-matched PET images. It is also difficult to have confidence in the accuracy. Rather, the probability that an incorrectly converted CT image makes the PET images distorted may be high.